I get soft reset when MTU is bigger than 65

[Stefan]

Hello!

I am trying to increase the MTU size to get higher speeds. When I increase the MTU above 65 the application locks/software reset when doing transfer. Up to 65 it works. I have increased RAM memory for SoftDevice a lot. No difference. I check the return value of softdevice_enable(). Always zero. I have not enabled DLE.

65 sounds awfully close to 64, but it is still a bit strange value.

Any hints or tips would be appreciated

[Sigurd]

Hi,

What SDK version are you using? What example in the SDK are you testing this on?

In e.g. SDK 13 in the ble_app_uart example you set the ATT_MTU with the nrf_ble_gatt_att_mtu_periph_set() function:

void gatt_init(void)
{
    ret_code_t err_code;

    err_code = nrf_ble_gatt_init(&m_gatt, gatt_evt_handler);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_ble_gatt_att_mtu_periph_set(&m_gatt, 64);
    APP_ERROR_CHECK(err_code);
}

[Sigurd]

General Note: A reset indicates that you are running into the error handler, where the default behavior is to do a reset. You should then debug in order to find the reason for the reset. See this post about debugging.

[Stefan]

Hi Sigurd,

I use SDK 12.2 (SD132).

I do not test on any particular example, I do test it on our application directly. I try to speed up transfers and currently increasing the MTU seems to be the only viable way.

I set MTU by:

#if (NRF_SD_BLE_API_VERSION == 3)
    ble_enable_params.gatt_enable_params.att_mtu = NRF_BLE_MAX_MTU_SIZE;
#endif
    // Enable BLE stack.
    ble_enable_params.gatts_enable_params.service_changed = IS_SRVC_CHANGED_CHARACT_PRESENT;
    err_code = softdevice_enable(&ble_enable_params);
    APP_ERROR_CHECK(err_code);

I hope this is the way to do it in 12.*.

I will check out the debugging post you refer to, thanks!

[Sigurd]

Are you handling the event BLE_GATTC_EVT_EXCHANGE_MTU_RSP ? and starting the ATT_MTU exhange with sd_ble_gattc_exchange_mtu_request() ? Take a look at the S132 v.3.0.0 migration document. "Configurable ATT_MTU" section contains information you need.

You can also find an example on how to enable DLE and increase ATT_MTU for SDK 12 in this post.

[Stefan]

Hi Sigurd,

your debug suggestion worked. I have now found that the error comes from softdevice_handler.c line 187 and I get NRF_ERROR_DATA_SIZE (error code 12). I do not understand why.

I will close this and open a new question, thanks for the help!

/Stefan